Are the gifts that I requested from my friends—who then traveled, and whom I will never see again—considered unlawful? And what is the expiation for them if they are?

If your friends give you what you asked for willingly, then there is no harm in possessing it, even if you were the one who requested it. This is permissible because it is allowed to request a gift from someone known to be willing to give it, and due to the special nature of friendship which permits eating from a friend's food without explicit permission. However, if you know that they gave you what they gave you out of embarrassment, then taking it becomes unlawful gain (suht) that must be returned to them. If it is impossible to return it, then give it as charity on their behalf.